This invention relates generally to the field of fluid drop generation and the application thereof to jet drop recorders of the type shown in Mathis, U.S. Pat. No. 3,701,998. In recorders of this type, there are a pair of rows of orifices in a plate which receive an electrically conducting recording fluid, such as for example a water base ink, from a pressurized manifold and eject the fluid, in the form of drops, in two rows of parallel streams. The fluid flows through these orifices and is formed into drops by the application of a series of traversing waves to the orifice-containing plate. This method of drop generation is more completely described in Lyons et al, U.S. Pat. No. 3,739,393, to which reference is made.
Graphic reproduction in recorders of this type is accomplished by selectively charging and deflecting some of the drops in each of the streams and thereafter disposing the uncharged drops on a moving web of paper or other material. Charging of the drops is accomplished by application of binary charge control singals to charging electrodes positioned near the tips of the fluid filaments immediately prior to drop formation. This induces an opposite charge upon the conductive fluid filaments, and a portion of this charge is carried away by the drops. Thereafter, the drops pass through electrostatic fields which have no effect upon the uncharged drops but which cause the charged drops to be deflected for catching by one or the other of a pair of catchers which service the rows of streams.
Cassill, U.S. Pat. No. 3,787,883, discloses apparatus for creating the deflecting electrostatic fields. A thin deflection ribbon is positioned between and parallel to the two rows of parallel drop streams with the catchers positioned outwardly of the drop streams. A voltage is applied between the deflection ribbon and the catchers such that charged ink drops will be deflected to one of the two catchers.
The apparatus generating the fluid drops may also generate droplets of small size which form an ink mist. While very little of this mist will be present at any one time, ink buildup on various surfaces of the printer, including the deflection electrodes, may result over a period of operation. An unwanted ink mist in the printer may also result from crooked ink jets or from difficulties encountered in starting up or shutting down the printer.
Heretofore, ink mist buildup on a deflection electrode has been remedied by mechanical clean-up of the electrode after ink has been deposited thereon. This was accomplished by periodically shutting down the printing operation and removing and cleaning the affected parts. However, such periodic cleaning was both inconvenient and expensive but necessary to avoid a deleterious ink buildup which would eventually affect printing quality. In response to the problem, various methods of continuously cleaning the deflection electrodes during operation were developed. Takano et al, U.S. Pat. No. 4,023,183, utilizes rotating cylindrically shaped deflection electrodes having cleaning pads located away from the electrical field through which the conductive ink drops pass. As the electrodes rotate, the pads continuously wipe away any ink which has been deposited thereon. Paranjpe, U.S. Pat. No. 4,031,563, and Chocholaty, U.S. Pat. No. 3,955,203, utilize a porous deflection ribbon connected to a vacuum source. Any ink deposited on the deflection ribbon is ingested into the ribbon and carried away by the vacuum. Another known technique is to provide grooves in deflection electrodes. Ink droplets deposited on the electrodes are drawn into the grooves and transported away from the electrode surface by virtue of the capillary action of the grooves as taught by Steffy, U.S. Pat. No. 3,813,675. Finally, Watanabe et al, U.S. Pat. No. 4,050,377, teach use of a heated element inside the aperture board of an ink mist printer to lower the relative humidity inside the board and prevent condensation.
Although all of the above methods function to a greater or lesser extent to remove ink once it has been deposited on a deflection electrode, it is highly desirable that ink be prevented from depositing on the electrode in the first place. If there is no ink deposition, there is no need to resort to the complication and expense of integrating a clean-up capability into the ink jet printer. Additionally, many of the prior art solutions to the ink mist deposition problem have resulted in a deflection electrode structure having a considerably increased cross-section. Because of the limited space available in a jet printer, particularly in the area occupied by the deflection electrode and catchers, it is highly desirable to maintain as thin a deflection electrode as possible.